AIR BALANCING HOODS: THE GO-TO TOOLS FOR MEASURING AIRFLOW

Many technicians attend our air balancing seminars expecting to learn how to use an air balancing hood and turn a damper. Sixteen training hours later, they comprehend that a balancing hood is only a single tool in an arsenal of HVAC diagnostic testing weapons, and that adjusting airflow is only the beginning.

Air balancing used to be a term reserved for adjusting high-rise commercial building HVAC systems. But today, in the residential and light commercial HVAC industry, the term air balancing is more accurately translated as HVAC system diagnostics, field-measured performance verification, or installed efficiency documentation.

While an air balancing hood is the most popular instrument used to verify the performance of an HVAC system, it can’t get the job done alone. To do the job right, it needs to work together with its test instrument cousins: the manometer, anemometer, thermometer, and psychrometer. Using these instruments together to verify system performance is an art, and requires a significant and sustained commitment to testing.

An air balancing hood in action.

First Encounters of the Bewildering Kind

The first encounters with an air balancing hood can leave a new tester bewildered. Most of us have built HVAC systems for years, and we’re certain our systems work perfectly. I’ve seen experienced technicians taking their first airflow test on a return grille expecting 1,000 cfm. I’ve watched them become dismayed, shake the hood, test again, and then curse the hood and walk away disgusted because it only read 600 cfm.

Initially, we all trust our systems and distrust our test instruments. Through sustained testing, the day comes when you read airflow and your first thought becomes “What’s wrong with my system?” rather than “What’s wrong with the hood?” We call this air balancing puberty. Welcome to the club.

Testing Challenges

One of the first challenges you’ll wrestle with is the constantly changing air volume through grilles. Steady state testing doesn’t exist in the field. In the HVAC industry, we’re in the business of heat transfer – moving heat from one place to another. The very nature of moving heat around means that testing will be affected by a constantly changing set of conditions. A register may vary from 190 cfm to 210 cfm within a few seconds. It’s a reality of how air moves through a system. Understanding constantly changing test conditions improves your confidence in your readings, and magnifies your ability to interpret them.

So, when the numbers jump around a little on your hood, don’t be discouraged. Either take an average of the readings, or consistently use the high reading and you’ll be just fine. Throughout it all, you’ll begin to understand how air really works.

One of the biggest challenges in hood testing is failure to get a tight seal between the hood and the wall, floor, or ceiling to which the grille is attached. Some registers require larger skirts to completely capture all the airflow. Various sizes are available, and you’ll probably have to be used from time to time. You’ll also use foam rubber adaptors to capture the airflow from floorboard grilles and other unusual installations.

Many registers pose testing challenges by their very nature. Toe-kick grilles cannot be read with a hood unless gaskets are used to direct the airflow from the register through the hood. However, rest assured that there are a number of methods that can be used to adapt a hood to gain consistent verifiable readings under adverse test conditions.

All Hoods Are Not Created Equal

Beware of some “disposable” hoods on the market that deliver unreliable test numbers. Because of their low cost (often around $1,000) they are frequently picked up to quickly balance a contractor’s first commercial job. They’re also widely used in low-income weatherization programs also. The bottom line with these hoods is that you get what you pay for. I recommend spending your money on a real hood that you can trust.

Good commercial hoods cost around $2,000 or more. However, a quality made hood, when used correctly, maintained well and kept calibrated, will deliver solid readings you can count on. It’s worth the investment.

The market offers a number of capture hoods. Discuss your purchase with a balancing tech who can recommend a reliable hood, and avoid the temptation to save a few bucks.

Quality air balancing hoods are extremely dependable, and are the go-to tool for those who have learned how to use them.

Keep in mind that obtaining good readings with any hood requires a working knowledge of measuring system performance and often demands additional testing to verify airflow readings taken with an air balancing hood using related system readings with other instruments.

That’s where we’ll pick up in the next installment of this series.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring and improving HVAC system performance. If you're an HVAC contractor or technician interested in a no cost one page test report and procedure on how to measure duct loss, contact Doc at robf@nationalcomfortinstitute.com or call him at 800-633-7058.

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